The invention concerns a method for predicting the acoustic and vibratory levels inside a vehicle rolling over ground that has several obstacles or ground of a given particle texture.
The discomfort perceived by the driver and passengers of a vehicle rolling over one or many obstacles (such as manholes, bitumen seams, various joints, gravel, etc.) has two distinct aspects. A first aspect is vibratory and is manifested as vibrations of the vehicle floor, the seats and the steering wheel. A second aspect is acoustic and is manifested as noise produced inside the vehicle by the vibrations of various parts of the vehicle. The discomfort level felt by the occupants of a vehicle depends greatly on the body, the mechanical system through which it is in contact with the ground, the rolling speed and of course the type of obstacle on the road.
Definition of the terms used in what follows:
the xe2x80x9ccomfort performancexe2x80x9d corresponds to the acoustic and/or vibratory level that can be measured inside a vehicle rolling on a road (or a test bed) provided with at least one obstacle of given dimensions;
xe2x80x9csuspension systemxe2x80x9d: the group of vehicle elements that provide the link between one or more contact surfaces between the vehicle and the ground and one or more points of the said vehicle; in all cases the suspension system comprises at least one tire and the wheel to which it is fitted;
xe2x80x9cbodyxe2x80x9d: the group of vehicle elements complementary to the suspension system;
xe2x80x9cattachment pointsxe2x80x9d: points connecting the suspension system with the body;
xe2x80x9coverall transfer finctionxe2x80x9d: a function comprising two parts, a first part concerning the noise in the cabin of the vehicle and a second part concerning the vibrations at certain predetermined points in the said cabin;
xe2x80x9creference suspension systemxe2x80x9d: suspension system fitted to the vehicle and available for obtaining the overall transfer function;
xe2x80x9cprototype suspension systemxe2x80x9d: suspension system whose effect on the comfort performance of the same vehicle fitted with the said prototype system in place of the reference system one is seeking to predict, without having the said vehicle available.
For example, a suspension system may consist of a tire and the wheel to which it is fitted: in this case, the point of attachment is the hub. The overall transfer function makes it possible to reproduce the vibration levels inside the body of a vehicle from the forces measured at the hub.
Several methods are known to those familiar with the field for evaluating the comfort performance of a new suspension system for a given vehicle, which make it possible to optimize the said suspension system, such optimization consisting in obtaining characteristics of a contact system that procure an appreciably improved comfort level.
For example, to evaluate and optimize the comfort performance of a vehicle fitted with a new suspension system, a person familiar with the field can use an experimental method involving measurements of the noise and vibrations in the cabin of a vehicle rolling on a section of road or track that produces some vibratory and/or acoustic discomfort in the said vehicle, this section of road or track having one or more obstacles on its surface. However, this method entails availability of the vehicle so that various suspension systems can be evaluated; besides, it can only be carried out in suitable weather conditions and this means that the vehicle is immobilized for times which are sometimes excessively long and consequently lead to excess costs during the development phase of a suspension system since several iterations are often needed. In addition, the method is long and fastidious for the operator and is subject to wide scatter of the measurements.
Furthermore, European Patent Application No. 886130 describes a method of predicting the noise level in the cabin of a vehicle fitted with tires and rolling over uneven ground that has numerous rough points. According to this method, a transfer function is determined for a vehicle fitted with tires by applying, directly to each axle (at the hub) of the vehicle at rest, forces (in the form of shocks) directed in predetermined directions. For each impact a sound recording is made inside the said vehicle and this operation is repeated successively for each of the front and rear positions and on each side of the vehicle. In another stage, an identical tire rolls on a roller track provided on its rolling surface with numerous rough points to simulate uneven ground. In this test, the tire is mounted on a fixed axle and the resultant forces acting at the hub are recorded. Finally, these measured resultant forces are used as input for a model involving the transfer function determined as described earlier so as to obtain the resultant noise level inside the vehicle. This method however, which is certainly interesting, has limitations which, for example with tires of the same size but having different structures, can give noise levels different from those obtained from tests carried out using the same vehicle fitted with these different tires and rolling on uneven ground.
In particular, since the transfer function of the vehicle is established from tests carried out at rest, it is clear that no account is taken of the mechanical characteristics of the tires when rolling, which as a general rule are appreciably different from the same characteristics at rest. Notably, it is known that the vertical rigidity of a tire at rest under dynamic loading is higher than the same vertical rigidity under dynamic loading when the tire is rolling.
Besides, to apply forces at the level of the axles it is necessary to provide an added, fixed component on the outside of the wheel, the said component being designed to receive, for example, blows from a hammer, and it is clear that the mass of the said component is added to the un-sprung weight and so perturbs the measurements made. It must also be pointed out that the noise of each hammer blow, even though means are adopted to attenuate it, is transmitted through the air into the cabin and is at least partly added to the noise one is trying to record inside the vehicle.
The object of the invention is a method of predicting the comfort performance of a vehicle fitted with a suspension system, which does not suffer from the drawbacks of the methods just described.
To achieve this, a method is proposed for predicting the noise and vibrations in the cabin of a vehicle fitted with a prototype suspension system, when the said vehicle is rolling at a given speed V on ground having at least one obstacle of predetermined size.
The method proposed comprises a first stage in which an overall transfer function is determined for the vehicle fitted with a reference suspension system, and a second stage in which forces are measured at the attachment points between the body and the prototype suspension system. Then, the method according to the invention consists in multiplying this overall transfer function of the vehicle obtained in the first stage by the resultant forces acting at the attachment points between the body and the prototype suspension system, when the said prototype suspension system is bearing on its contact surfaces with the ground the same loads as those measured when the vehicle is rolling on the same ground provided with the same obstacle(s).
The overall transfer function is determined by using a series of measurements on the vehicle fitted with a reference suspension system combined with a series of measurements on this reference suspension system attached to a frame at the body attachment points whereby it is fitted to a vehicle. In the case when the suspension system is reduced to the tire and the wheel, the frame is equipped with means that enable the measurement of forces acting at the hub.
The measurements on the vehicle comprise the following stages:
a) the vehicle is fitted with the reference suspension system, the said system being connected to the said vehicle by attachment points;
b) inside the vehicle are arranged means that can record the noise and vibrations at previously predetermined points in the cabin;
c) the vehicle is positioned such that the reference suspension system is in contact via each tire of the said system with rolling means provided on the rolling surface(s) with an obstacle or obstacles;
d) each tire positioned on the said rolling means is rotated at speed V and the noise and vibration signals inside the cabin are recorded.
Then, the resultant forces acting at the said attachment points are obtained for this reference suspension system by carrying out the following steps:
e) the reference suspension system is mechanically fixed to a frame by the same attachment points, the said frame being equipped at these points with means whereby the resultant forces can be determined in three mutually perpendicular directions;
f) the said reference suspension system is applied against the means of rolling used at stage c) provided with the same obstacle(s), such that at its contact surface(s) with the ground the said suspension system supports loads identical to those supported on the vehicle in the position considered;
g) each tire positioned on the rolling means is rotated at speed V and the resultant forces at the attachment points to the frame are recorded.
The combination of the values measured both on the vehicle fitted with the reference suspension system and on the reference suspension system when fitted to a frame, makes it possible to obtain the overall transfer function for the said vehicle rolling on ground provided with an obstacle or obstacles identical to that/those used in the said measurements.
In the case when the suspension system is reduced to the tire and the wheel to which it is fitted, stages e) to g) are carried out again under conditions appropriate for the other axle of the same vehicle, and stages c) and d) are repeated for all the other front and rear positions on the vehicle which have not yet been measured.
It is conceivable to replace the measurements carried out on the reference suspension system (stages e, f, g) by a numerical simulation with the help of a functional model that represents the said system and makes it possible to obtain the forces acting at the body attachment points in a configuration similar to that of rolling over obstacles.
After determining the overall transfer function, the forces acting at the body attachment points of the same vehicle fitted with a prototype suspension system in place of the reference suspension system are measured, reproducing stages e), f) and g) identically with the said prototype suspension system fixed to the same frame and in the same way as the reference suspension system tested previously.
These forces measured at the body attachment points of the prototype suspension system are then multiplied by the overall transfer function of the vehicle to obtain the noise and vibration levels inside the cabin and so to allow characterization of the comfort performance of the vehicle fitted with the prototype suspension system.
As mentioned for the reference suspension system, it is also conceivable to replace the measurements on the prototype suspension system (stages e, f, g) by a numerical simulation with the help of the same functional model, making it possible to obtain the forces that act at the attachment points of the prototype suspension system to the frame in a similar configuration of rolling over obstacles.
The method according to the invention makes it possible to evaluate the comfort performance of a new (prototype) suspension system fitted to a vehicle initially equipped with a known (reference) suspension system. In addition, it is no longer necessary for the vehicle to be available in order to evaluate a new suspension system different from the reference suspension system, once the overall transfer function for the vehicle has been established using the said reference system on a rolling means, provided that the same rolling means is used for the measurement or numerical simulation of the forces acting at the attachment points of the prototype suspension system when fitted to the frame.
To obtain a transfer function still more representative of the behavior of the vehicle at the speed V chosen, it is advantageous to repeat stage d) with the vehicle and stages e), f) and g) with the suspension system alone with at least two different speeds which bracket the speed V. Preferably, two speeds are chosen on either side of the speed V, these being about 10% and 35% higher and lower than V. This makes it possible to take account of non-linearities of the vehicle as a function of the speed value V chosen. The overall transfer function is then obtained by calculating the average of the transfer function established for each speed.
In the case when the suspension system considered is reduced to a tire and the wheel to which it is fitted, it has been found, surprisingly, that for a given vehicle the tire size is immaterial. This means that for a given vehicle designed to be fitted with various tire sizes (for example, tires having different H/S ratios where H is the width of the tire and S the height of its cross-section), an overall transfer function can be determined that applies for all the tire sizes accepted by the said vehicle. To do this, the procedure described above is followed through steps a) to d) for the vehicle fitted with reference tires and steps e) to g) are carried out with the same reference tire, but making all the measurements with at least two other tire inflation pressures. Preferably, the pressures used are between 10% and 20% higher than the utilization pressure of the reference tire and 10% to 20% lower than the utilization pressure of the reference tire. This procedure makes it possible to take account of any variation of rigidity related to the tire""s size. The overall transfer function is then obtained by calculating the average of the transfer functions determined for each tire pressure and each speed.
With the method according to the invention, it is easy to predict the comfort performance of as many prototype suspension systems as desired. For each suspension system, the cost of this prediction will amount to the cost of measuring the forces at the body attachment points of the said suspension system, or the cost of numerical simulation of the said forces.
By choosing to proceed in this way, one can appreciate for example all the advantages offered by the method for developing a suspension system for a prototype vehicle, which can only be made available for a short time. Thanks to the method according to the invention, one can even envisage in the long term the possibility of data exchange between a manufacturer of suspension systems and a vehicle manufacturer, without the suspension system manufacturer being able to have access to the vehicle, since the vehicle manufacturer can undertake to test the vehicle and then provide the said vehicle""s overall transfer function to the suspension system manufacturer, specifying the conditions under which it was obtained.
Another advantage of the method according to the invention is that the transfer function in effect integrates the noise component resulting from the actual impact of the tire(s) on the obstacle(s) and the impact noise is indeed the real noise.
A further advantage is that it is in effect the real forces acting at the attachment points which are taken into account when determining the overall transfer function and in its subsequent use to evaluate and optimize a new suspension system.
Needless to say, the method according to the invention can be applied to any type of vehicle, notably touring vehicles, vans or heavy-goods vehicles in which comfort performance is important.
The prediction method according to the invention easily allows the size and number of obstacles on the rolling means to be adapted in order to simulate various types of road surface.
Finally, it should be pointed out that the information yielded by the transfer function determined by the method according to the invention makes it possible effectively to improve the comfort performance of a suspension system, since in fact it distinguishes between the transfer of vibrations via the body from the transfer of vibrations via the suspension system.